The non-melanoma skin cancers (NMSCs), basal and squamous cell carcinoma are the most common cancers, with approximately 1 million new cases diagnosed per year. Although the NMSCs, in particular basal cell carcinomas (BCCs) are, for the most part, essentially curable, surgical treatment often results in scarring and disfigurement, and the risk of recurrence is high (the chance of a second occurrence is 35-50%). The major risk factor for the development of NMSCs is chronic sun exposure (ultraviolet irradiation) and age. BCCs, which make up approximately 80% of the NMSCs, arise from basal keratinocytes in the epidermis of the skin. Studies in our laboratory have provided multiple lines of evidence suggesting that in these keratinocytes, a signaling enzyme called protein kinase D (PKD), in particular PKD1, promotes keratinocyte proliferation and inhibits differentiation. Thus, we have shown in published results that (1) a panel of inhibitors that selectively target PKD promote keratinocyte differentiation, (2) keratinocyte differentiating agents decrease PKD expression/activation, as if to allow differentiation to proceed, (3) PKD is expressed in the proliferating basal layer in the intact epidermis in mice and humans and (4) over-expression of PKD increases the promoter activity of a proliferative marker, keratin 5, and inhibits the promoter activity of a differentiation marker, involucrin. We have also demonstrated that PKD is up-regulated in human BCCs and mislocalized in the hyper-proliferative skin disorder psoriasis. Recently, we have also obtained data to support the idea that PKD is activated upon exposure of keratinocytes to ultraviolet B irradiation. Thus, these results suggest a link between sun exposure/ultraviolet light, PKD levels/activation and human skin diseases, such as BCC. Finally, a recent report using RNA interference to knock down PKD1 expression in human keratinocytes demonstrated a reduction in proliferation in response to this manipulation, providing further support for our hypothesis that PKD is a pro-proliferative and/or anti-differentiative signal in keratinocytes. In the experiments proposed here, we intend to test this idea using conditional PKD1 knockout mice. We will obtain these mice from their creator, Dr. Eric Olson, and examine the effects of PKD1 loss on keratinocyte proliferation and differentiation marker expression/levels in vitro (using adenovirus expressing Cre recombinase to delete the floxed PKD1 gene) and in vivo (using a commercially available mouse model in which the keratin 14 promoter drives expression of Cre recombinase to generate epidermal-specific PKD1 knockout mice). The results of these studies should allow us to begin to determine the role of PKD1 in epidermal structure and function. Ultimately, this research may allow the identification of novel molecules, such as PKD1 (or its downstream effectors), which may be targeted to treat and/or prevent hyper-proliferative skin diseases. PUBLIC HEALTH RELEVANCE: Skin diseases afflict millions of Americans each year. Indeed, two disorders, the non-melanoma skin cancers and psoriasis, are both extremely common, with new diagnoses of non-melanoma skin cancers surpassing those for all other cancers combined and psoriasis affecting approximately two percent of the population. Diseases such as the non-melanoma skin cancers and psoriasis are characterized by excessive growth of the primary cells comprising the outer layer of the skin, the epidermal keratinocytes. We have obtained data to indicate that protein kinase D is a signaling enzyme in epidermal keratinocytes, which promotes cell growth and/or inhibits maturation. In order to test this idea, we will use genetically engineered mice in which the gene for protein kinase D is knocked out to drastically decrease its levels. We will then determine the effects of this decrease on cell growth and maturation both in isolated keratinocytes in culture and in the skin of animals. This research should provide insight into the role of protein kinase D in skin structure and function and allow identification of new targets for the development of drugs to treat human skin diseases.